1. Field of the Invention
The present invention relates to a substrate for mounting an electronic part, allowing a conductive pin to be inserted and secured in a through hole firmly, a method for producing such substrate and the conductive pin and particularly, to a structure of the conductive pin.
2. Description of the Related Arts
Referring to FIGS. 16 and 17, a conventional substrate 9 for mounting an electronic part is composed of an insulating substrate 90, a recessed space 95 in a center of the insulating substrate 90 where an electronic part is mounted and a frame-like dam 98 formed around the periphery of the recessed space 95. Reference numerals 93 and 94 designate a conductive circuit and a land, respectively.
A head 921 of a conductive pin 92 is inserted into a through hole 91 so that the head 921 is electrically connected to a plating layer 911 coated with an inner wall of the through hole 91. The conductive pin 92 is provided with a collar 922 and a leg 923. The conductive pin 92 inserted into each corner of the insulating substrate 90 is further provided with a lower collar 924.
Referring to FIG. 17, the head 921 of the conductive pin 92 is bonded to the through hole 91 by soldering of a solder 8 in order to reinforce the electric bonding and further to provide mechanical strength between the conductive pin 92 and the through hole 91.
The soldering is executed by using a reflow method as shown in FIG. 17. That is, the solder 8 melted into a molten state is supplied in the direction opposite to the insertion of the conductive pin 92.
More specifically, as shown in FIG. 18, the head 921 of the conductive pin 92 is inserted into the through hole 91 of the insulating substrate 90, on which a solder paste 81 formed of solder particles, flux or the like is placed and then heated for melting. The molten solder 8 flows into a solder gap defined by the through hole 91 and the head 921 for bonding therebetween (See FIG. 17).
The above-described substrate 90 for mounting an electronic part is of a face down type, in which the conductive pin is inserted from the same surface where the recessed space 95 for mounting an electronic part is formed.
In the above conventional art, the smaller the diameter of the head 921 of the conductive pin 92 becomes, the easier the head 921 can be inserted into the through hole 91. In case the diameter of the head 921 is too small, the conductive pin 92 is likely to fall out from the through hole 91 in the middle of the soldering process. While in case the diameter of the head 921 is too large for tight fitting, the conductive pin 92 cannot be fully inserted, failing to have the collar 922 abutted on the land 94, or an inner wall of the through hole 91 might be cracked or the plating layer coated with the inner wall surface of the through hole 91 might be peeled off because of strong pressure exerted to the inner wall during insertion.